Air can be separated into its component parts by several techniques, the most common of which are cryogenic distillation, membrane separation and adsorption.
Recent advances in adsorption technology have made this method of air separation highly suitable for a variety of applications, e.g. portable oxygen or nitrogen generators. The particular adsorbent used in air separation depends, in general, upon the product that is sought. It is usually preferred to use an adsorbent that will adsorb the unwanted components from the gas being separated, so that the desired product can be obtained as the non-adsorbed, high pressure product. Thus, when oxygen is the desired product, nitrogen-selective adsorbents, such as sodium- or lithium-exchanged type X zeolite or calcium-exchanged type A zeolite are used; and when nitrogen is the desired product, it is more efficient to use an oxygen-selective adsorbent.
One of the more preferred oxygen-selective adsorbents is carbon molecular sieve (CMS), since nitrogen of very high purity, e.g. 99% purity, can be produced using this adsorbent. CMS adsorbs both oxygen and nitrogen; however, it adsorbs oxygen at a considerable faster rate than it adsorbs nitrogen. Hence, CMS can be efficiently used for oxygen-nitrogen separations carried out by pressure swing adsorption (PSA) processes having very short cycles. The adsorption is generally carried out at temperatures below 50.degree. C., since best results are obtained at low temperatures.
Japanese Kokai Patent No. Hei 5(1993)-4044 describes a process for the removal of oxygen from oxygen-containing gas by a temperature swing adsorption process using as the adsorbent a doped perovskite compound having the formula ABO.sub.x, where A is selected from Sr, La, Ba, Pb and Ca, and B is selected from Co, Fe and Zr, the perovskite being doped with metals other than those listed for A and B. The adsorption step of the described process is carried out at a temperature in the range of 0 to 900.degree. C., and the desorption step of the process is carried out at a temperature of 20 to 900.degree. C.
It is sometimes desirable to separate oxygen from oxygen-containing gas streams by PSA at temperatures above 100.degree. C. or even above 300.degree. C. by PSA techniques. Furthermore, it would be highly desirable to provide a PSA process in which the adsorbed component is oxygen and it is obtained at very high purity. The present invention provides an oxygen-adsorbing PSA process that possesses these advantages.